Food Calorimetry: Common Mistakes

Well, I want to talk about two of the most

common

mistakes

people make when doing

food

calorimetry

problems. Well, this is what I mean by

food

calorimetry

. Let's say we want to know how many calories a French fry has. We will take this fried potato and set it on fire under a container of water. A lot of times, if you're doing this in the lab, the water container you use is like a soda can, so the burnt potato chip will be released. heat and that's going to heat the water in this container, then after it burns for a while the temperature of the water increases, so to calculate how many calories of thermal energy are in this potato chip, we measure the temperature difference from the beginning. to finish and then we plug that number, as well as others, into this equation q = m * C * delta T.

So, this is what I mean by food calorimetry. Now I want to go over a practice problem that might be helpful even if you're I don't make any of these

mistakes

, but I want to break down this practice problem and show you where people make

common

mistakes while I solve this problem. I'm going to intentionally make a mistake to see if you can catch it. Some students plan to do so. they measure the amount of calories in a French fry they fill a metal can this container here with 60 ml of water at 15° because they take a 1.9 gram French fry and set it on fire under the can that's what's happening here After the potato chip has stopped burning, the temperature of the water has risen to 85° cus, so the question is how many calories are in the chip, since you probably know that we are going to use this equation, the first thing we are going to do is solve is delta T, okay, so delta.

T is the final temperature minus the initial temperature, so the final temperature here is 85° C 85° C minus minus the initial temperature, which is 15° C, that's what we start with, okay, so if we subtract this , we will find that delta T is equal to 85 minus 15, which is 70° C, okay, now we have delta T, let's plug some numbers into this equation so we can say that Q is equal to m, we want a mass that will be 1.9 g times C, which is the specific heat which is 1 calorie divided by G*Dees C, this is the number of calories it takes to take 1 gram of water and increase its temperature by 1°C, so 1 calorie during the play time of c times delta T 7 ° C is equal to multiplying these three numbers together and I get 30 calories and I will also point out that my units are canel I have grams up here grams down there uh degrees Celsius down here degrees Celsius up there okay, makes sense?

Totally wrong, this is probably the most common mistake people make. When they ask these types of questions, do you know what's wrong with it? look at this equation here is what's wrong with it is this number right here 1.9 G 1.9 gr is the mass of the potato chip but I don't want to use the mass of the potato chip I want to use the mass of the water here is why let's look back what's really going on here okay, so we have this potato chip, we set it on fire so it releases its heat energy okay, so it gives off all of its heat energy and then the container of water on top of the potato chip which absorbs all the energy that the potato fry is releasing, so if we want to calculate how much energy the fry released, we have to What we do is calculate how much thermal energy the water absorbed, so that's what we're looking for, the amount of thermal energy absorbed by the water, to That's what we're using this equation.

Okay, but let's look more specifically at the variables. Q is the heat absorbed by water, which will tell us how much heat the potato chip gave off, well Q, heat absorbed by water, so it's all about the water, we're focusing on the water here. M has to be the mass of the water because we are focusing on the water. Well, C is sometimes written as s is the specific heat of the water, how much energy is needed to raise the temperature of this water and finally the delta T is a temperature change of the water, so it's all about the water, It is about the amount of heat absorbed by the water, so we have to use the mass of water that is in this container, we cannot use the mass of the potato chip because this is all about water, you must use the mass of the water, It's okay, I think.

This can be complicated because of the way the problem was written. Well, here's why I bet you look at this equation and think, huh M. I'm going to need a mass mass the mass is in grams oh look, there's a mass. right here 1.9 G. I better put that in the equation. Okay, notice that the problem does not immediately give you a mass for the water. Okay, so it only gives you 60 milliliters of water. That's not a mass, it's a volume. So I think sometimes people take a look at these issues and say oh 1.9G, that's what I want to use, so this brings us to the second big mistake that people make.

I don't give you a mass for water in this question, how can you solve it? this is how water is very special and it is good because it is easy for water and only for water 1 ml is equivalent to one gr so here I did not give the mass of water but I gave milliliters and as you know that 1 ml is equivalent to 1 G this is very easy to realize that 60 MERS of water is equivalent to 60 G of water okay then we can replace 1.9 G of the fried potato mass that was wrong with 60 G which is the mass of water okay keep in mind that it is just water that has 1 mil is equal to 1 G it's not any liquid it's just water water special it's very easy to remember this is okay, anyway 60 G times the specific heat time delta T I can cancel my grams will give me a much larger number. to give me 4200 calories, okay, keep in mind that if you do these calculations to find the number of calories in the French fry, these are what we call Little C calories, they are not food calories, also known as kilocalories or calories written with a large letter c so you need to divide this number by a th to get the number of calories in the food, but it's still 4,200 little calories C.

Let's go over the two things we talked about here. The first thing we said is that when doing this equation q = m * C * delta T, you are solving for the amount of heat absorbed by the water given off by the burning potato chip or whatever, a Cheeto or a burning Frito, it doesn't matter. . Well, you're in either of those situations and you're trying to calculate the heat absorbed by the water, which means you have to use the mass of the water, not the mass of whatever is burning. Well, that's the first thing, body of water. it has to be your M.

The second thing is that if they don't give you a mass of water, if they just give you milliliters of water, it's very easy to calculate what the masses are and that's because a milliliter of water is a gram of water. This is how you can always tell what the mass of water is even if it is not given in the problem, so keep these two things in mind and you will avoid the two most common mistakes people make when solving these types of problems. Good luck.